The monthly commitment is a minimum level of spend - not a membership fee. If you generally spend that amount or more in a month, you will be paying only for the trips you have made.
If in a particular month you have spent less than your monthly commitment, and you are on a $60 or higher monthly commitment, your unused spend will rollover to the following month.
If you exceed your committed amount in any month, you will simply be charged for excess usage at the standard time/distance rates applicable to your plan.